IN 2006, WHEN people of all fitness levels got their first and, until tomorrow, only chance to run through Dublin’s Port Tunnel, it appeared that they were at risk of being outnumbered by the cranes on the horizon.

Buildings were still being built and the boom that Bertie had spoken of was, by all accounts, still getting boomier.

The only light at the end of the tunnel that was on the minds of the near 10,000 runners who ran that day was the literal one, which came at both the five kilometre mark – when participants exited the North tunnel onto the N1, before entering the southbound tunnel – and the 10 kilometre mark.

Fast forward six years and the phrase has since taken on an all too figurative meaning, with many of tomorrow’s runners at the Dublin Port Tunnel run hoping to see some light at the end of their own personal tunnels.

Before that, however, they have a run to complete unlike any other.

While the luxury of having near constant cover may sound like the perfect solution to Ireland’s weather, running in a tunnel creates an altogether different problem in the form of body heat and recycled air aplenty.

Tomorrow’s Dublin Port Tunnel run is set to add an extra level of difficulty for those who find mini-marathons all too achievable, while at the same time raising funds for Focus Ireland.

For those who can’t make it (you can register right up until tomorrow morning), the below video offers an insight into what it felt like back in 2006, complete with the obligatory Olé, Olé, Olé at the end.

(ricmurphy/YouTube)

If that video has left you feeling inspired but still unavailable, all that lies between you and a similar experience is a plane ticket and some spontaneity.

Liverpool, England

The Queensway Tunnel, which runs under Liverpool’s River Mersey, forms part of the course for this year’s Liverpool marathon which takes place on 14 October.

The Queensway Tunnel in Liverpool, England. (Eric The Fish (2012)/Flickr)

A previous runner of the Liverpool marathon, TheJournal.ie's own Hugh O'Connell, describes the toughest section of the 26 mile run.

One of the most difficult legs of the course was going through the tunnel. It runs for around 3.2 kilometres, which is just over two miles and is dark, damp and not very pleasing on the eye but then what underwater tunnels are?

The large enclosed space can start to feel isolated, as the noise generated by the outside world gradually fades away.

The first thing I noticed when I took the descent into it - literally descending down under the water - is that there was less noise than what I had just passed which was screaming crowds as people lined the streets to urge the runners on. There was lots of that on the streets of Birkenhead but once I was in the tunnel there was none of that as the noise slowly disappeared the further and further I got into the tunnel.

And let's not forget the dead air.

It felt like it went on forever, the end was never in sight and the air was dead, with no breeze, just the sound of panting and shoes hitting the ground and a few people talking to each other as they struggled through.

As I completed the first mile and went into the second I could see the light and yet it was so far away. It certainly felt a lot closer than it actually was and that second mile was definitely slower than the first one.

Maryland, USA

For those with more air miles than miles left in the tank, the Fort McHenry Tunnel awaits where, on 16 September, you can pop your tunnel running cherry in their five kilometre run/walk.

The Fort McHenry Tunnel, Baltimore, Maryland. (dbking/Flickr)

What has been your most extreme challenge (and no, extreme ironing doesn't count)?